Why Do My Discriminator Taps Always Stop Working Eventually?

[CanesFan95]

Everytime I do a discriminator tap mod, it works fine for like 2-3 years. But then, something happens and it quits putting out discriminator audio. I literally have 3 different scanners modded and all 3 have done the exact same thing. A BC15X, BC780XLT, and a BCD996P2. It's been the same pattern everytime. All 3 worked great after doing the mod, and kept working great for a couple years.

But one-by-one, they all gradually died out. I've tried troubleshooting and can no longer get discriminator audio out the computer speakers. I just get silence now. But the scanners all run and receive just fine. So what would cause this? Has anyone had a similar issue?

 

[ladn]

Are your solder connections at the radio deteriorating? What happens if you redo the discriminator tap with new parts?

 

[CanesFan95]

I tried poking with bare wires to get audio to the computer speakers and never could get audio. So I don't know if re-doing the mod would work.

 

[a417]

Do you have the audio present at the location at which you tapped?

 

[ka3aaa]

how are you making the tap, there are several different ways to do it.

 

[CanesFan95]

Do you have the audio present at the location at which you tapped?

I basically get no audio from the discriminator output at all. If I plug into the external speaker jack, I can get audio that I can hear out the computer speakers. I have a PSR-800 with the built-in discriminator output turned on and it works fine, for comparison.

Here's an old thread of how I did the mod:

Where To Solder On A Stereo 3.5mm Jack For Discriminator Tap?

I no longer have the PRO-2006, but I basically did the tap the same way on the Unidens. The way I did it was a wire soldered to the tap point plus a resister and capacitor to the 3.5mm jack. Then another wire from a ground screw to another prong on the jack. Drilled a hold to mount the jack and that's it.

I just wonder if there's something I'm doing that slowly fries a component somewhere and breaks it.

 

[GTR8000]

Perhaps it's a hint that it's time to stop using discriminator tapped scanners, and buy a few dongles instead. You'll get much better decoding results with software like DSD+, Unitrunker, SDRTrunk, etc.

 

[CanesFan95]

well, I do have multiple dongles and 2 AirSpies on DSD+ Fast Lane, but I like a conventional scanner the best for searching around the bands and decoding new frequencies. Scanners work a lot better for that with the old school knobs and buttons. It's fun to search a band with a discriminator scanner and find new frequencies.

 

[jonwienke]

well, I do have multiple dongles and 2 AirSpies on DSD+ Fast Lane, but I like a conventional scanner the best for searching around the bands and decoding new frequencies. Scanners work a lot better for that with the old school knobs and buttons. It's fun to search a band with a discriminator scanner and find new frequencies.

LOL. A SDR dongle can search 10MHz of bandwidth in less than a second--way faster than any standard scanner. And software like SDR# can automatically log all your hits by frequency. Using a scanner to search a frequency range when you have SDRs is like pushing a Ferrari by hand instead of starting the engine and driving it.

 

[ka3aaa]

but scanners are not computer dependent like sdr's are and require computers to run 24/7

 

[jonwienke]

but scanners are not computer dependent like sdr's are and require computers to run 24/7

They're 10x slower, though. So hardly the best tool for the job.

 

[JoshuaHufford]

Have you checked the values of the resistor or capacitor to see if they have drifted out of spec?

Are you using really cheap components that are failing?

It can't be that complicated, either a connection has failed or a component has failed. Break out your multimeter and you should be able to answer your own question.

 

[prcguy]

I see in the link you mention using a Tantalum capacitor. Those can be trouble and will blow up if you look at them for too long. They were popular starting in the 70s because they packed a lot of capacitance for their size, but they have become the main failure point in most radios made in the 70s, 80s and 90s until they were not used any longer by reputable mfrs.

I think in many scanners the discriminator tap is a point in the scanner where you tap off raw information, otherwise the same information flows on downstream in the scanner audio circuits and out the speaker. In other words, if the internal discriminator circuit was broken you would not get audio out of the scanners internal speaker, so I think the problem is in the components you added.

I would check the series capacitor that taps off the discriminator and also measure if there is any DC in the circuit. If the Tantalum cap is in the circuit backwards and there is the slightest DC, it will not live long. In fact, toss out the Tantalum cap and put in another type of cap like a mylar or similar that is not polarized.

 

[slicerwizard]

Other capacitor types will not have the required capacitance.

 

[n5ims]

Other capacitor types will not have the required capacitance.

10 mf 16v (based on the pic in the linked article)? A very common value capacitor. Here's one in electrolytic that is very similar to the tantalum. -->

https://www.mouser.com/ProductDetail/Panasonic/ECE-A1CKA100I?qs=sGAEpiMZZMvwFf0viD3Y3ffldXPLozay0HkrKwlXULs%3D

Or if you want a non-polarized cap as stated above, try this -->

https://www.mouser.com/ProductDetail/TDK/FK11X5R1C106K?qs=%2Fha2pyFaduiChU%2FquvA7OU9RKuEa3v2Is49SDwKE9Is%3D

 

[Ubbe]

The only thing that can fail are the capacitor. Put a new one in and perhaps try it polarized the other way. Some audio inputs on a PC are made for powering capacitor mic's and might output up to 5volts and the discriminator tap probably have lower voltage than that.

Or use a non polarized cap. They are much bigger but shouldn't be a problem. A 1uF cap have a reactance of 1k ohm at 150Hz and the input of a PC are probly higher than that, so should be ok to use lower capacitance when decoding digital signals without screwing up the signal too much.

Electronics 2000 | Reactance Calculator

Electronics 2000, for electronics hobbyists, engineers and students. Online calculators, software downloads, technical data, beginners guide, pinouts, forum, links and more

www.electronics2000.co.uk

/Ubbe

 

[slicerwizard]

A 1uF cap have a reactance of 1k ohm at 150Hz and the input of a PC are probly higher than that, so should be ok to use lower capacitance when decoding digital signals without screwing up the signal too much.

And what's its reactance at say, 10 Hz? Digital signals need good response (low reactance) down to nearly DC.

 

[jonwienke]

And what's its reactance at say, 10 Hz? Digital signals need good response (low reactance) down to nearly DC.

No. Low frequencies have very little bandwidth, so there's very little to be gained using frequencies below 100Hz.

 

[mancow]

I see in the link you mention using a Tantalum capacitor. Those can be trouble and will blow up if you look at them for too long. They were popular starting in the 70s because they packed a lot of capacitance for their size, but they have become the main failure point in most radios made in the 70s, 80s and 90s until they were not used any longer by reputable mfrs.

I think in many scanners the discriminator tap is a point in the scanner where you tap off raw information, otherwise the same information flows on downstream in the scanner audio circuits and out the speaker. In other words, if the internal discriminator circuit was broken you would not get audio out of the scanners internal speaker, so I think the problem is in the components you added.

I would check the series capacitor that taps off the discriminator and also measure if there is any DC in the circuit. If the Tantalum cap is in the circuit backwards and there is the slightest DC, it will not live long. In fact, toss out the Tantalum cap and put in another type of cap like a mylar or similar that is not polarized.

That appears to be what burned up in my Harris.

 

[slicerwizard]

No. Low frequencies have very little bandwidth, so there's very little to be gained using frequencies below 100Hz.

So you think this P25 signal has no low frequency content? Then how does it manage to stay negative for so long?